If (when?) quantum computing manages to break public key cryptography, we're going to have a lot more problems than just crypto-currencies being potentially hacked.

Of course that's assuming nothing changes. I would guess when quantum computing eventually does become more mature that we'll see a shift from current cryptography standards to ones that are more resistant against quantum computing, like we're already starting to see in some cases (such as Iota). While the transition may or may not be easy, I would expect cryptos and the internet at large to adapt over time before it starts to become a major problem.